Issues...

So, I'm not really sure where to post this so here it goes. I have a 4 year old and almost 2 year old. Lately, my 4 year old thinks that it's okay to hit, push, kick, throw toys at, my youngest. The worst of it is when he holds her face down into the furniture (couch, bed, anything soft). I don't know if this is just a faze or if there's something more at play. He's around absolutely no violence of any kind, there's been no changes to anything, nothing obvious that could spark this behaviour. He is almost double her weight and size, so it's not like she's able to push him off when needed. I'm at such a loss, I don't know what to do... I guess what I'm asking is, has anyone else gone through this? If so, how did you stop it?